Method for producing laser-engravable flexographic printing elements on flexible metallic supports

ABSTRACT

The invention relates to a method for producing laser-engravable flexographic printing elements on flexible metallic supports comprising a cross-linked elastomeric layer and an absorber for laser radiation. The invention also relates to a method for producing flexographic printing plates by means of laser engraving using flexographic printing elements of the aforementioned type, and to flexographic printing plates produced using such a method.

[0001] The present invention relates to a process for the production oflaser-engravable flexographic printing elements on flexible metallicsupports which comprise a crosslinked elastomeric layer comprising anabsorber for laser radiation. The present invention furthermore relatesto a process for the production of flexographic printing plates by meansof laser engraving using flexographic printing elements of this type,and to flexographic printing plates produced by a process of this type.

[0002] The conventional method for the production of flexographicprinting plates by laying a photographic mask on a photopolymericrecording element, irradiating the element with actinic light throughthis mask, and washing the unpolymerized areas of the exposed elementout using a developer liquid is increasingly being replaced by methodsin which lasers are used.

[0003] In laser direct engraving, recesses are engraved directly into asuitable elastomeric layer with the aid of a laser of sufficiently highpower, in particular by means of an IR laser, forming a relief which issuitable for printing. To this end, large amounts of the material ofwhich the printing relief consists have to be removed. A typicalflexographic printing plate is nowadays, for example, between 0.5 and 7mm in thickness and the non-printing recesses in the plate are between300 μm and 3 mm in depth. The method of laser direct engraving for theproduction of flexographic printing plates therefore only attractedcommercial interest in recent years with the appearance of improvedlaser systems, although laser engraving of rubber printing cylindersusing CO₂ lasers has basically been known since the late 1960s. Thedemand for suitable laser-engravable flexographic printing elements asstarting material for the production of flexographic printing plates bymeans of laser engraving has thus also increased significantly.

[0004] In principle, commercially available photopolymerizableflexographic printing elements can be employed for the production offlexographic printing plates by means of laser engraving. U.S. Pat. No.5,259,311 discloses a process in which, in a first step, theflexographic printing element is photochemically crosslinked byirradiation over the full surface and, in a second step, a printingrelief is engraved in by means of a laser. However, the sensitivity offlexographic printing elements of this type to CO₂ lasers is low, and inaddition a post-washing step for the removal of residues is necessary.

[0005] It has therefore been proposed, for example in EP-A 640 043 andEP-A 640 044, to admix substances which absorb IR radiation with theelastomeric layer to be laser-engraved in order to increase thesensitivity. However, substances of this type, such as carbon black orcertain dyes, also absorb very strongly in the UV/VIS region.Flexographic printing elements which comprise these absorbers thereforecan at best be photochemically crosslinked in a very thin layer, or notat all. Thus, EP-A 640 043 discloses the production of a carbonblack-containing, elastomeric layer by photocrosslinking. However, thislayer only has a thickness of 0.076 mm, while the typical thickness ofcommercially available flexographic printing plates is from 0.5 to 7 mm.

[0006] Flexographic printing plates are employed, inter alia, for thefinishing of sheet-fed offset print products, for example by varnishingor gold printing (see, for example, “Inline-Veredelung überFlexo-Lackierwerke” [In-Line Finishing by Means of Flexo VarnishingMachines] Deutscher Drucker 29 (1999) w2 w6). Flexographic printingplates intended for this purpose are therefore also known as varnishplates. Particular importance is attached to registration accuracy inthis area. Modern flexographic varnishing units in sheet-fed offsetmachines are frequently equipped with quick-action clamp bars or withfully automatic plate feed devices which are only suitable for the feedof printing plates having a metallic support. In order to be suitablefor this application, commercially available flexographic printingplates on PET supports are therefore bonded to an additional aluminumsupport. This requires an additional working step, which istime-consuming and labor-intensive. It is therefore desirable to producelaser-engravable printing elements directly on a metallic support.

[0007] It is an object of the present invention to provide a simple andeconomical process for the production of laser-engravable flexographicprinting plates on metallic supports.

[0008] We have found that this object is achieved by a process for theproduction of laser-engravable flexographic printing elements, at leastcomprising a flexible metallic support and a crosslinked elastomericlayer which comprises at least one absorber for laser radiation, whichcomprises the following steps:

[0009] (a) preparation of a thermally crosslinkable mixture by intimatemixing of at least one elastomeric binder, at least one absorber forlaser radiation and at least one polymerization initiator in a suitablesolvent,

[0010] (b) application of the mixture to a temporary support,

[0011] (c) evaporation of the solvent at a temperature T₁,

[0012] (d) lamination of the dried layer by means of the side facingaway from the support onto a flexible metallic support,

[0013] (e) optionally removal of the temporary support, and

[0014] (f) thermal crosslinking of the polymerizable layer by warming toa temperature T₂, where T₂ is at least 80° C. and T₂ is greater than T₁.

[0015] In a further embodiment, the invention relates to a furtherprocess for the production of laser-engravable flexographic printingelements of this type which comprises the following steps:

[0016] (a) preparation of a thermally crosslinkable mixture by intimatemixing of at least one elastomeric binder, at least one absorber forlaser radiation and at least one polymerization initiator in a suitablesolvent,

[0017] (b) application of the mixture to a flexible, metallic support,

[0018] (c) evaporation of the solvent at a temperature T₁,

[0019] (d) thermal crosslinking of the dried, polymerizable layer bywarming to a temperature T₂, where T₂ is at least 80° C. and T₂ isgreater than T₁.

[0020] The invention furthermore relates to a process for the productionof flexographic printing plates by engraving a printing relief by meansof a laser into the flexographic printing elements obtained by theprocess according to the invention, and to flexographic printing platesobtained by the process.

[0021] The following details apply to the process according to theinvention.

[0022] The flexographic printing elements obtained by the processaccording to the invention comprise a laser-engravable, crosslinkedelastomeric layer on a flexible metallic support.

[0023] The term “laser-engravable” is taken to mean that the layer hasthe property of absorbing laser radiation, in particular the radiationfrom an IR laser, in such a way that it is removed or at least loosenedat the points at which it has been subjected to a laser beam ofsufficient intensity. The layer is preferably evaporated without priormelting or decomposed thermally or oxidatively, so that itsdecomposition products are removed from the layer in the form of hotgases, vapors, smoke or small particles. However, the invention alsocovers subsequently removing the residues of the irradiated layer bymechanical means, for example by blasting off with a liquid or a gas oralso, for example, by suction or wiping off using a cloth.

[0024] The metallic flexographic printing elements support employed forthe process according to the invention are flexible. For the purposes ofthe present invention, the term flexible is taken to mean that thesupports are so thin that they can be bent around printing cylinders. Onthe other hand, however, they are also dimensionally stable andsufficiently thick that the support is not kinked during production ofthe flexographic printing element or mounting of the finished printingplate on the printing cylinder.

[0025] Suitable flexible metallic supports are in particular thin sheetsor foils of steel, preferably of stainless steel, magnetizable springsteel, aluminum, zinc, magnesium, nickel, chromium or copper, it alsobeing possible for the metals to be alloyed. It is also possible toemploy combined metallic supports, for example steel sheets coated withtin, zinc, chromium, aluminum, nickel or also combinations of variousmetals, or also metal supports obtained by lamination of identical ordifferent metal sheets. It is furthermore also possible to employpretreated sheets, for example phosphated or chromatized steel sheets oranodized aluminum sheets. In general, the sheets or foils are degreasedbefore use. Preference is given to sheets made from steel or aluminum.Particular preference is given to magnetizable spring steel.Flexographic printing plates on supports of this type can be clampeddirectly onto magnetic printing cylinders without adhesive tapes or thelike.

[0026] The thickness of flexible metal supports of this type is usuallyfrom 0.025 mm to 0.4 mm and depends, besides on the desired degree offlexibility, also on the type of metal employed.

[0027] Supports made from steel usually have a thickness of from 0.025to 0.30 mm, in particular from 0.14 to 0.24 mm. Supports made fromaluminum usually have a thickness of from 0.25 to 0.4 mm.

[0028] The flexible metal support is advantageously provided with ananchor layer which is insoluble and swelling-resistant in printing inks.The anchor layer promotes good adhesion between the flexible, metallicsupport and the laser-engravable layer to be applied later, so that thelatter does not detach on bending around the laser drum or around theprinting cylinder.

[0029] It is in principle possible to employ any anchor layer in orderto carry out the present process, provided that the anchor layer isinsoluble and swelling-resistant in the organic solvents or solventscontaining organic components which are usual in flexographic printinginks, for example ethanol or isopropanol.

[0030] An anchor layer which has proven suitable for carrying out theprocess according to the invention is, for example, one which comprisesa binder embedded in a suitable polymeric matrix. In general, discretedomains of elastomeric binder and the matrix are evident under themicroscope.

[0031] Examples of suitable binders for the anchor layer includeelastomeric and thermoplastic elastomeric polymers which are usuallyalso employed for the production of relief printing plates, such aspolymers or copolymers of 1,3-dienes or SIS or SBS block copolymers. Itis also possible to employ mixtures of two or more different elastomericbinders.

[0032] The amount of elastomeric binder in the anchor layer isdetermined by the person skilled in the art depending on the desiredproperties. It is usually from 10 to 70% by weight, based on the sum ofall components of the anchor layer, in particular from 10 to 45% byweight and very particularly from 15 to 35% by weight.

[0033] The polymeric matrix is usually a crosslinked polymeric matrixobtainable by means of a suitable crosslinking system. The crosslinkedpolymeric matrix can be obtained thermally by polycondensation orpolyaddition of suitable monomers or oligomers, for example by reactionof polyisocyanates and suitable hydroxyl-containing compounds, such ashydroxyl-containing polyurethane resins or polyester resins, withformation of crosslinked polyurethanes.

[0034] If desired, the anchor layer may contain further components andauxiliaries, for example additional binders for influencing theproperties, dyes, pigments or plasticizers.

[0035] For the production of the anchor layer, the binder and thefurther components of the anchor layer are usually dissolved in suitablesolvents, for example THF, toluene or ethyl acetate, and mixedvigorously with one another, and the solution is filtered if necessaryand applied to the flexible metallic support. The application can takeplace, for example, by means of a roll or by means of a caster. Afterthe application, the solvent is evaporated, and the system issubsequently crosslinked. The residual solvent content in the layershould be less than 5% by weight, based on all constituents of thelayer.

[0036] The thickness of the anchor layer is usually from 2 to 100 μm,preferably from 10 to 50 μm and particularly preferably from 15 to 30μm. It is also possible to employ a plurality of anchor layers ofidentical, approximately identical or different composition one on topof the other.

[0037] The outlined anchor layer firstly promotes good adhesion betweenthe laser-engravable layer and the flexible metallic support and isinsoluble and non-swellable in organic solvents usually used forflexographic printing inks. In addition, they have particularly goodfreedom from tack. This is particularly advantageous if the metallicsupports are not processed further immediately after coating. Metallicsupports coated in this way can be stacked or rolled during productionwithout additional measures, for example insertion of paper asinterlayer, without sticking together. The invention naturally alsocovers in-line application of an anchor layer.

[0038] For the production of the laser-engravable elastomeric layer, anintimate mixture of at least one elastomeric binder, at least onepolymerization initiator and at least one absorber for laser radiationis prepared in a suitable solvent in one process step. The mixture mayin addition comprise ethylenically unsaturated monomers and furtherauxiliaries and/or additives.

[0039] The elastomeric binders employed can be the known binders usuallyused for the production of photopolymerizable flexographic printingplates. In principle, both elastomeric binders and thermoplasticelastomeric binders are suitable. Examples of suitable binders are theknown three-block copolymers of the SIS or SBS type, which may also befully or partially hydrogenated. It is also possible to employelastomeric polymers of the ethylene-propylene-diene type,ethylene-acrylic acid rubbers or elastomeric polymers based on acrylatesor acrylate copolymers. Further examples of suitable polymers aredisclosed in DE-A 22 15 090, EP-A 084 851, EP-A 819 984 or EP-A 553 662.The polymeric binders may contain crosslinkable groups, for exampleethylenically unsaturated groups, in the main chain of the polymer. Itis also possible to employ binders containing crosslinkable side groups.

[0040] It is also possible to employ mixtures of two or more differentbinders.

[0041] The type and amount of the binder employed are selected by theperson skilled in the art depending on the desired properties of theprinting relief. In general, the amount of binder is from 50 to 95% byweight, based on the amount of all constituents of the dried,laser-engravable layer, i.e. without taking into account the solvent.The amount is preferably from 60 to 90% by weight.

[0042] The recording layer according to the invention furthermorecomprises at least one absorber for laser radiation. It is also possibleto employ mixtures of different absorbers for laser radiation. Suitableabsorbers for laser radiation have high absorption in the region of thelaser wavelength. Particularly suitable absorbers are those which havehigh absorption in the near infra-red and in the longer-wave VIS regionof the electromagnetic spectrum. Absorbers of this type are particularlysuitable for the absorption of the radiation from high-power Nd:YAGlasers (1064 nm) and from IR diode lasers, which typically havewavelengths between 700 and 900 nm and between 1200 and 1600 nm.

[0043] Examples of suitable absorbers for the laser radiation are dyeswhich absorb strongly in the infra-red spectral region, for examplephthalocyanines, naphthalocyanines, cyanines, quinones, metal complexdyes, for example dithiolenes, or photochromic dyes.

[0044] Further suitable absorbers are inorganic pigments, in particularintensely colored inorganic pigments, for example chromium oxides, ironoxides, carbon black or metallic particles.

[0045] Particularly suitable absorbers for laser radiation are finelydivided carbon black grades having a particle size of from 10 to 50 nm.

[0046] Further particularly suitable absorbers for laser radiation areiron-containing solids, in particular intensely colored iron oxides.Iron oxides of this type are commercially available and are usuallyemployed as colored pigments or as pigments for magnetic recording.Suitable absorbers for laser radiation are, for example, FeO, goethiteα-FeOOH, akaganeite β-FeOOH, lepidocrocite γ-FeOOH, hematite α-Fe₂O₃,maghemite γ-Fe₂O₃, magnetite Fe₃O₄ or berthollide. It is furthermorepossible to employ doped iron oxides or mixed oxides of iron with othermetals. Examples of mixed oxides are umbra Fe₂O₃×n MnO₂ orFe_(x)Al(_(1-x))OOH, in particular various spinel black pigments, forexample Cu(Cr, Fe)₂O₄, Co(Cr, Fe)₂O₄ or Cu(Cr, Fe, Mn)₂O₄. Examples ofdopants are P, Si, Al, Mg, Zn and Cr. Dopants of this type are generallyadded in small amounts during the synthesis of the oxides in order tocontrol the particle size and particle shape. The iron oxides may alsobe coated. Coatings of this type can be applied, in order to improve thedispersibility of the particles. These coatings may consist, forexample, of inorganic compounds, such as SiO₂ and/or AlOOH. However, itis also possible to apply organic coatings, for example organic adhesionpromoters, such as aminopropyl(trimethoxy)silane. Particularly suitableabsorbers for laser radiation are FeOOH, Fe₂O₃ and Fe₃O₄, veryparticularly preferably Fe₃O₄.

[0047] The size of the iron-containing, inorganic solids employed, inparticular the iron oxides, is selected by the person skilled in the artdepending on the desired properties of the recording material. However,solids having a mean particle size of greater than 10 μm are generallyunsuitable. Since iron oxides, in particular, are anisometric, thisdimension refers to the longest axis. The particle size is preferablyless than 1 μm. It is also possible to employ so-called transparent ironoxides, which have a particle size of less than 0.1 μm and a specificsurface area of up to 150 m²/g.

[0048] Further iron-containing compounds which are suitable as absorbersfor laser radiation are iron metal pigments. Particularly suitable areneedle-shaped or rice grain-shaped pigments having a length of from 0.1to 1 μm. Pigments of this type are known as magnetic pigments formagnetic recording. Besides iron, further dopants, such as Al, Si, Mg,P, Co, Ni, Nd or Y, may also be present, or the iron metal pigments maybe coated therewith. Iron metal pigments are partially oxidized on thesurface for protection against corrosion and consist of a doped orundoped iron core and a doped or undoped iron oxide shell.

[0049] At least 0.1% by weight of absorber, based on the sum of allconstituents of the laser-engravable elastomeric layer, is employed. Theamount of absorber added is selected by the person skilled in the artdepending on the respective desired properties of the laser-engravableflexographic printing element. In this connection, the person skilled inthe art will take into account that the absorbers added affect not onlythe rate and efficiency of the engraving of the elastomeric layer bylaser, but also other properties of the flexographic printing element,for example its hardness, elasticity, thermal conductivity, abrasionresistance and ink take-up. In general, therefore, more than 20% byweight of absorber for laser radiation, based on the sum of allconstituents of the laser-engravable elastomeric layer, is unsuitable.The amount of absorber for laser radiation is preferably from 0.5 to 15%by weight and particularly preferably from 0.5 to 10% by weight.

[0050] Polymerization initiators which can be employed are in principlecommercially available thermal initiators for free-radicalpolymerization, for example peroxides, hydroperoxides or azo compounds.

[0051] The choice of suitable initiators has particular importance forcarrying out the process according to the invention. Suitable thermalinitiators do not decompose into free radicals at high reaction rateuntil the final step of the process according to the invention, thethermal crosslinking. They are substantially thermally stable in thepreceding process steps of mixing and dispersion, casting, evaporationof the solvent and lamination. The term “thermally substantially stable”in this connection means that the initiators decompose at most so slowlyduring performance of these steps of the process according to theinvention that crosslinking of the layer and/or of the mixture bypolymerization can only take place to a secondary extent, and does notimpair the orderly performance of the process.

[0052] The thermal stability of an initiator is usually indicated bymeans of the temperature of the 10 hour half life 10 h-t_(1/2), i.e. thetemperature at which 50% of the original initiator amount has decomposedto form free radicals after 10 hours. Further details in this respectare give in “Encyclopedia of Polymer Science and Engineering”, Vol. 11,pages 1 ff., John Wiley & Sons, New York, 1988. Particularly suitableinitiators for carrying out the process according to the inventionusually have a 10 h-t_(1/2) of at least 60° C., preferably of at least70° C. Particularly suitable initiators have a 10 h-t_(1/2) of at least80° C.

[0053] Particularly suitable initiators for carrying out the processaccording to the invention usually have a 10 h-t_(1/2) of at least 60°C,, preferably of at least 70° C. Particularly suitable initiators havea 10 h-t_(1/2) of at least 80° C.

[0054] Examples of suitable initiators include certain propoxy esters,such as t-butyl peroctanoate, t-amyl peroctanoate, t-butylperoxyisobutyrate, t-butyl peroxymaleate, t-amyl perbenzoate, di-t-butyldiperoxyphthalate, t-butyl perbenzoate, t-butyl peracetate and2,5-di(benzoylperoxy)-2,5-dimethylhexane, certain diperoxyketals, suchas 1,1-di(t-amylperoxy)cyclohexane, 1,1-di(t-butylperoxy)cyclohexane,2,2-di(t-butylperoxy)butane and ethyl 3,3-di(t-butylperoxy)butyrate,certain dialkyl peroxides, such as di-t-butyl peroxide, t-butyl cumeneperoxide or 2,5-di(t-butylperoxy)-2,5-dimethylhexane, certain diacylperoxides, such as dibenzoyl peroxide or diacetyl peroxide, certaint-alkyl hydroperoxides, such as t-butyl hydroperoxide, t-amylhydroperoxide, pinane hydroperoxide and cumene hydroperoxide. Alsosuitable are certain azo compounds, for example 1-(t-butylazo)formamide,2-(t-butylazo)isobutyronitrile, 1-(t-butylazo)cyclohexanecarbonitrile,2-(t-butylazo)-2-methylbutanenitrile, 2,2′-azobis(2-acetoxypropane),1,1′-azobis(cyclohexanecarbonitrile), 2,2′-azobis(isobutyronitrile) and2,2′-azobis(2-methylbutanenitrile).

[0055] From 1 to 15% by weight, preferably from 1 to 10% by weight, ofinitiator, based on the amount of all constituents of thelaser-engravable layer, are usually employed.

[0056] The process according to the invention can be carried out byusing only the ethylenically unsaturated groups present as side groupsor in the main chain of the binder for the crosslinking. However, it isalso possible to employ in addition ethylenically unsaturated monomers.The ethylenically unsaturated monomers employed can basically be thoseusually also employed for the production of photopolymerizableflexographic printing elements. The monomers should be compatible withthe binders and have at least one polymerizable, ethylenicallyunsaturated double bond. Esters and amides of acrylic acid ormethacrylic acid with mono- or polyfunctional alcohols, amines,aminoalcohols or hydroxyethers and -esters, styrene or substitutedstyrenes, esters of fumaric or maleic acid or allyl compounds haveproven particularly advantageous. Examples of suitable monomers arebutyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 1,4-butanedioldiacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate,1,9-nonanediol diacrylate, trimethylolpropane triacrylate, dioctylfumarate, N-dodecylmaleimide. It is also possible to employ mixtures ofdifferent monomers. The type and amount of the monomer is determined byperson skilled in the art depending on the desired properties and thebinder employed. In general, however, the total amount of the monomersis not greater than 30% by weight, based on the amount of allconstituents of the laser-engravable layer, and preferably not greaterthan 20% by weight.

[0057] If desired, further additives and auxiliaries, for exampleplasticizers, fillers, dyes, compatibilizers or dispersion auxiliaries,can be employed in order to set the desired properties of the relieflayer. The amount of such further constituents should, however,generally not exceed 20% by weight, preferably 10% by weight.

[0058] The constituents for the production of the laser-engravable layerare intimately mixed with one another in a suitable solvent, giving ahomogeneous mixture or dispersion of the constituents. In general, it isadvisable to dissolve all organic constituents of the layer ascompletely as possible, and to disperse inorganic constituents, forexample carbon black or iron oxide pigments as absorbers for laserradiation, uniformly in the organic matrix.

[0059] A suitable solvent is selected by the person skilled in the artdepending on the constituents used in the layer. Suitable solventsinclude, in particular, toluene, xylenes, cyclohexane and THF. It isalso possible to employ mixtures of different solvents.

[0060] The intimate mixing of the constituents can be carried out atroom temperature or also at temperatures above room temperature. Theperson skilled in the art will ensure that he selects a temperature forthe dissolution process which is matched to the boiling point of thesolvent and the 10 h-t_(1/2) of the initiator. In general, the mixingshould not be carried out at temperatures above 60° C. The intimatemixing can be carried out using conventional stirring or dispersionunits. If necessary, the solution can be filtered before use.

[0061] In the first embodiment of the process according to theinvention, the mixture is applied to a temporary support. Suitabletemporary supports are, in particular, PET films, which, in order tosimplify later peeling-off, may also be modified, for example bysiliconization. The application is generally carried out by means of aroll or caster, the thickness adjustment of the layer being carried outby means of parameters known in principle to the person skilled in theart, such as adjustment of the casting gap, take-off rate and/orviscosity of the solution. After application, the solvent is evaporatedat a temperature T₁. The evaporation of solvent can be carried out, forexample, in a drying tunnel. The temperature T₁ can be selected by theperson skilled in the art depending on the desired circumstances, forexample the boiling point of the solvent, the desired drying rate or thedesired residual solvent content. In general, T₁ is greater than 25° C.T₁ is preferably between 30° C. and 80° C. and is for example 40° C.However, it is also possible to select temperatures above 80° C. inparticular cases. In order to avoid premature polymerization, however,the temperature T₁ is in all cases lower than the temperature T₂ atwhich thermal crosslinking is effected in a later process step. Theresidual solvent content in the layer after the drying operation shouldbe less than 5% by weight, based on all constituents of the layer. Theresidual solvent content is preferably less than 3% by weight, based onthe sum of all constituents of the layer.

[0062] It is also possible to cast a plurality of laser-engravablelayers of identical, approximately identical or different compositionone on top of the other. In principle, casting can be carried out eitherwet-on-wet, or the respective lower layer can firstly be partially driedor fully dried before the second layer is cast on.

[0063] Furthermore, it is possible, if desired, to cast additionallayers, which take on other tasks in the system, and their compositiontherefore differs from that of the laser-engravable layer(s).

[0064] For example, it is possible to cast a thin upper layer whichforms the printing surface in the finished flexographic printing plate.An upper layer of this type allows parameters which are essential forthe printing behavior and ink transfer, such as roughness, abrasiveness,surface tension, surface tack, ink take-up or solvent resistance, on theprinting surface to be modified without the relief-typical properties ofthe printing plate, for example hardness or elasticity, being affected.Surface properties and layer properties can thus be modifiedindependently of one another in order to achieve an optimum printresult. The upper layer may comprise an absorber for laser radiationwithout this being absolutely necessary. The composition of the upperlayer is only restricted inasmuch as the laser engraving of thelaser-engravable layer beneath it must not be impaired and the upperlayer must be removable together therewith. The upper layer should bethin compared with the laser-engravable layer. The thickness of an upperlayer of this type should generally not exceed 100 μm, with thethickness preferably being between 5 and 80 μm, particularly preferablybetween 10 and 50 μm.

[0065] It is furthermore also possible to cast a thermallypolymerizable, but not laser-engravable underlayer located between thesupport and the laser-engravable layer in the finished flexographicprinting element. By means of underlayers of this type, the mechanicalproperties of the relief printing plates can be modified withoutaffecting the relief-typical properties of the printing plate.

[0066] The dried, thermally polymerizable layer or the multilayer systemof corresponding layers is laminated with the side facing away from atemporary support onto the flexible metallic support using a suitablesolvent. An example of a suitable lamination solvent is tetrahydrofuran.

[0067] After the lamination, it is advisable to peel off the temporarysupport in order to prevent potential complications owing to shrinkageor excessive adhesion of the support to the laser-engravable layerduring the crosslinking, although this is not absolutely necessary inevery individual case.

[0068] In the final process step for the production of the flexographicprinting element according to the invention, the polymerizable layer isthermally crosslinked by warming to the temperature T₂. The temperatureT₂ is at least 80° C. and is above T₁. The difference between T₁ and T₂is determined by the person skilled in the art depending on the specificcircumstances. In general, a difference of at least 10° C. is advisable,preferably a difference of at least 20° C. and particularly preferably adifference of at least 30° C. Larger differences, for example those of50° C., can also be selected. In general, T₂ is between 80° C. and 180°C., preferably between 80° C. and 150° C. and particularly preferablybetween 90° C. and 130° C. For example, T₂ is 100° C.

[0069] The thickness of the crosslinked, elastomeric layer or of themultilayer system is generally from 0.1 to 7 mm, preferably from 0.5 to5 mm. A suitable thickness is selected by the person skilled in the artdepending on the desired application of the printing plate.

[0070] If the laser-engravable flexographic printing element no longerhas a temporary support, it can, if desired, be protected by aprotective film, for example a PET film, which is laid or laminated ontothe surface.

[0071] In a further embodiment of the process according to theinvention, the laser-engravable layer is not cast onto a temporarysupport, but instead directly onto the flexible metallic support, whichmay, if desired, have been coated with an anchor layer. The laminationstep is thus superfluous.

[0072] The laser-engravable flexographic printing elements obtained bythe process according to the invention serve as starting material forthe production of flexographic printing plates.

[0073] The process comprises firstly peeling off any protective filmpresent. In the following process step, a printing relief is engravedinto the flexographic printing element by means of a laser. Pixels whoseedges initially drop vertically and only widen in the lower region areadvantageously engraved. This results in good support of the image dots,but nevertheless low dot gain. However, it is also possible to engraveimage dot edges of a different shape.

[0074] Particularly suitable for laser engraving are Nd:YAG lasers (1064nm), IR diode lasers, which typically have wavelengths of from 700 to900 nm and from 1200 to 1600 nm, and CO₂ lasers having a wavelength of10640 nm. However, it is also possible to employ lasers of shorterwavelength, provided that the lasers have adequate intensity. Forexample, a frequency-doubled (532 nm) or frequency-trebled (355 nm)Nd:YAG laser can also be employed. Laser equipment of this type iscommercially available. The image information to be engraved istransferred directly from the layout computer system to the laserequipment. The lasers can be operated either continuously or in pulsedmode.

[0075] The laser engraving can advantageously be carried out in thepresence of an oxygen-containing gas, in particular air. Theoxygen-containing gas can be blown over the recording element during theengraving. A comparatively gentle gas stream can be generated, forexample, with the aid of a fan. However, it is also possible to blow astronger stream over the recording material with the aid of a suitablenozzle. This embodiment has the advantage that detached solidconstituents of the layer can be removed effectively.

[0076] The flexographic printing plate obtained can, if desired,subsequently be cleaned. A cleaning step of this type removes layerconstituents which have been detached, but not yet been completelyremoved from the plate surface. The printing plate can, for example, becleaned using a brush. This cleaning process can be supported by asuitable aqueous and/or organic solvent. A suitable solvent is selectedby the person skilled in the art with the proviso that it must notdissolve or greatly swell the relief layer. However, the cleaning canalso be carried out, for example, using compressed air or by vacuum.

[0077] The process according to the invention gives flexographicprinting plates on metallic supports which have been produced by laserengraving and are distinguished by excellent dimensional stability. Theyare particularly suitable for use in flexographic varnishing units ofsheet-fed offset printing machines.

[0078] The examples below are intended to explain the invention ingreater detail without representing a limitation.

[0079] Experimental:

[0080] For the laser engraving experiments, the laser-engravableflexographic printing elements were stuck to the cylinder of an ALElaser machine (Meridian Finesse) by means of an adhesive tape. Thismachine is fitted with an Nd:YAG laser having a power of 130 W. Afteradjusting the focus to the plate thickness, the plate was exposed to thelaser radiation at a rate of 160 cm/s and a feed rate of 20 μm.

EXAMPLE 1

[0081] A mixture of the following components was prepared in toluene ata temperature of 30° C.: Proportion by weight [%] (without ComponentStarting Material toluene) Binder Kraton 1161, SIS block copolymer, 77Shell α-Methylstyrene-vinyltoluene copolymer 8 Piccotex 100, HerculesMonomers Hexanediol diacrylate 7 Hexanediol dimethacrylate 4 CarbonPrintex U, Degussa 1 black Initiator tert-Butyl peroctanoate(10h-t_({fraction (1/2+L )}) 3 73° C.

[0082] The components were dissolved, and the carbon black was dispersedtherein. The homogeneous dispersion obtained was degassed and coatedonto a PET film as temporary support (Lumirror ×43, 150 μm) by means ofa chamber caster. After drying (2 hours at 40° C., fan-assisted), thedry layer thickness was 950 μm. This layer was bonded by lamination to ametallic support (steel, thickness 0.14 mm) coated with adhesivelacquer. The film was subsequently peeled off. The dried layer wasthermochemically crosslinked by warming at 100° C for 45 minutes.

[0083] Laser engraving:

[0084] The laser-engravable flexographic printing element obtained wasengraved by means of lasers as described above. A relief depth of 460 μmwas obtained. The resolution was 60 lines/cm.

EXAMPLE 2

[0085] The mixture obtained in Example 1 was cast directly onto ametallic support (steel, thickness 0.05 mm) coated with an adhesivelacquer by means of a chamber caster. The layer was dried at 40° C. for2 hours. The dried layer was thermochemically crosslinked by warming at100° C. for 45 minutes.

[0086] Laser engraving:

[0087] The laser-engravable flexographic printing element obtained wasengraved by means of lasers as described above. A relief depth of 460 μmwas obtained. The resolution was 60 lines/cm.

EXAMPLE 3

[0088] A mixture of the following components was prepared in toluene ata temperature of 30° C.: Proportion by weight [%] (without ComponentStarting material toluene) Binder EPDM rubber Keltan 1446A, DSM 77α-Methylstyrene-vinyltoluene 8 copolymer Piccotex 100, Hercules MonomersHexanediol diacrylate 7 Hexanediol dimethacrylate 4 Carbon Printex U,Degussa 1 black Initiator Tert-Butyl peroctanoate 3(10h-t_({fraction (1/2+L )})73° C.)

[0089] The components were dissolved, and the carbon black was dispersedtherein. The homogeneous dispersion obtained was degassed and coatedonto a PET film as temporary support (Lumirror ×43, 150 μm) by means ofa chamber caster. After drying (2 hours at 40° C., fan-assisted), thedry layer thickness was 950 μm. This layer was bonded by lamination to ametallic support (steel, thickness 0.14 mm) coated with adhesivelacquer. The film was subsequently peeled off. The dried layer wasthermochemically crosslinked by warming at 100° C. for 45 minutes.

[0090] Laser engraving:

[0091] The laser-engravable flexographic printing element obtained wasengraved by means of lasers as described above. A relief depth of 530 μmwas obtained. The resolution was 60 lines/cm.

EXAMPLE 4

[0092] The mixture obtained in Example 3 was cast directly onto ametallic support (steel, thickness 0.05 mm) coated with an adhesivelacquer by means of a chamber caster. The layer was dried at 40° C. for2 hours. The dried layer was thermochemically crosslinked by warming at100° C. for 45 minutes.

[0093] Laser engraving:

[0094] The laser-engravable flexographic printing element obtained wasengraved by means of lasers as described above. A relief depth of 540 μmwas obtained. The resolution was 60 lines/cm.

1. A process for the production of laser-engravable flexographicprinting elements, at least comprising a flexible metallic support and acrosslinked elastomeric layer which comprises at least one absorber forlaser radiation, which comprises the following steps: (a) preparation ofa thermally crosslinkable mixture by intimate mixing of at least oneelastomeric binder, 0.5 to 20% by weight of at least one absorber forlaser radiation and at least one polymerization initiator whose 10h-t_(1/2) 10 hour half-life temperature is at least 60° C., in asuitable solvents (b) application of the mixture to a temporary support,(c) evaporation of the solvent at a temperature T₁, (d) lamination ofthe dried layer by means of the side facing away from the support onto aflexible metallic support, (e) optionally removal of the temporarysupport, and (f) thermal crosslinking of the polymerizable layer bywarming to a temperature T₂, where T₂ is at least 80° C. and T₂ isgreater than T₁, the thickness of the crosslinked elastomeric layerbeing from, 0.5 to 5 mm.
 2. A process for the production oflaser-engravable flexographic printing elements, at least comprising aflexible metallic support and a crosslinked elastomeric layer whichcomprises at least one absorber for laser radiation, which comprises thefollowing steps: (a) preparation of a thermally crosslinkable mixture byintimate mixing of at least one elastomeric binder, 0.5 to 20% by weightof at least one absorber for laser radiation and at least onepolymerization initiator whose 10 h-t_(1/2) 10 hour half-lifetemperature is at least 60° C., in a suitable solvent, (b) applicationof the mixture to a flexible, metallic support, (c) evaporation of thesolvent at a temperature T₁, (d) thermal crosslinking of the dried,polymerizable layer by warming to a temperature T₂, where T₂ is at least80° C. and T₂ is greater than T₁, the thickness of the crosslinkedelastomeric layer being from 0.5 to 5 mm.
 3. A process as claimed ineither of claims 1 and 2, wherein the thermally crosslinkable mixturefurthermore comprises at least one ethylenically unsaturated monomer. 4.A process as claimed in any one of claims 1 to 3, wherein the thermallycrosslinkable mixture comprises further additives and auxiliaries.
 5. Aprocess as claimed in any one of claims 1 to 4, wherein the flexible,metallic support is a support made from aluminum, steel or magnetizablespring steel.
 6. A process as claimed in any one of claims 1 to 5,wherein the flexible metallic support is provided with an adhesivelayer.
 7. A process as claimed in any one of claims 1 to 6, wherein theamount of the absorber for laser radiation is from 0.5 to 10% by weight,based on the amount of all constituents of the crosslinked, elastomericlayer.
 8. A process as claimed in any one of claims 1 to 7, wherein theabsorber for laser radiation is carbon black and/or an iron-containing,inorganic solid.
 9. A process for the production of flexographicprinting plates, which comprises engraving a relief into alaser-engravable flexographic printing element produced by a process asclaimed in claims 1 to 8, by means of a laser.
 10. A flexographicprinting plate produced by a process as claimed in claim
 9. Process forthe production of laser-engravable flexographic printing elements onflexile metallic supports